Forecasting tax increases, conservatively at first (5)
I believe the answer is:
vatic
'forecasting' is the definition.
(vatic means predicting the future)
'tax increases conservatively at first' is the wordplay.
'tax' becomes 'VAT' (UK Value Added Tax).
'at first' indicates taking the first letters.
The first letters of 'increases conservatively' is 'ic'.
'vat'+'ic'='VATIC'
(Other definitions for vatic that I've seen before include "Inspired" , "Predicting" , "about prophecy" , "Prophetic" , "as a prophet" .)